This invention relates to materials and methods for soldering semiconductor devices developed for optical communication and optical information process applications.
In response to the demand for increasing semiconductor laser applications, significant developments of GaAs and InP system laser devices in the fields of crystal growth, processing, and packaging technologies have been accomplished. Among these, the advancement of packaging technology contributed significantly to reduce its cost and to improve the overall performance and reliability as those laser devices are mass-produced.
In order to reduce their thermal resistance, optical semiconductor devices such as a semiconductor laser device are conventionally packaged by a construction shown in FIG. 8 wherein 1 is a heat-sink made of a high thermal-conductivity material such as copper, and 2 is a soft metal-solder such as indium, employed to relieve mechanical stress caused by a difference between respective thermal expansion coefficients of semiconductor device 50 and heat sink 1.
This packaging method is referred to as a direct bonding method, and in this manner, a low thermal resistance on the order of 40.degree.C/W can be easily obtained when a semiconductor laser is solder mounted on a heat sink.
However, this direct bonding method calls for a highly controlled thickness and flatness of the indium solder layer. This means that the bonding strength could be inadequate if the solder layer is too thin. If the solder layer is too thick, electrical short-circuit or leakage could occur by creeping of indium solder layer 2, which may cause bridging of the pn-junction 9 exposed on a side of semiconductor device 50 as shown in FIG. 9.
Furthermore, if there are any irregularities on the surface of indium solder layer 2, inadequate soldering of semiconductor device 50 on a heat sink 1 would result due to a possible gap which tends to inhibit the heat transfer between semiconductor device and heat sink 1. Therefore, in order to solve these problems, the indium layer 14 has to be formed by vacuum deposition for attaining an exact thickness of 2 to 3 microns.
However, the heat sink on which the indium deposition has to be made is integrated with a stem 8. Any part which should be excluded from the indium deposition, such as a wiring post 7, has to be individually masked, and this complicates the production process considerably.
In order to solve these problems, a method of forming solder layer 2 shown in FIG. 10 was previously disclosed by the Japanese Laid-Open Patent Sho 62-143496. According to the disclosed method, the indium layer 2 is formed first on a film 20, and in order to transfer this indium layer onto a heat sink, this layer is pressed against a preheated heat sink 1 by applying a pressure force and ultrasonic energy produced by a tool 6 from the reversed side of the film 20 as shown in FIG. 10. This method contributed considerably to simplifying the required soldering process and to reducing the production costs.
Whereas the film employed in the above-described direct bonding method has to be Teflon.RTM. (polytetrafluoroethylene) film because of its low flexural elastic constant and high mechanical durability under the applied ultrasonic energy used to melt the indium layer onto the heat-sink and to transfer it completely thereto, dissociation of carbon and fluorine from the Teflon surface is inevitable. This is proved by a result of Auger-Electron Spectroscopy (AES) as shown in FIG. 12, conducted on the surface of a transferred indium layer. The spectral peaks of carbon, fluorine, and other elements are attributable to those elements left on the transferred indium layer, and these are considered responsible for inadequate soldering strength.
Furthermore, with this direct bonding method, difficulties had been experienced in soldering a semiconductor device 5 having an irregular surface such as shown in FIG. 13. This is because gap 15, produced between semiconductor device 5 and heat sink 1 due to the irregular surface, can not be filled by thin indium layer 2 which has a thickness of 2 to 3 microns.